There's one interesting property of white dwarfs that I haven't
mentioned.
If a white dwarf exceeds a certain mass, about 1.4 
, it is no
longer able to support itself by electron degeneracy pressure.
This limit
is called the Chandrasekhar limit, after the physicist who
derived it in
terms of fundamental constants. A white dwarf that exceeds this
mass must
continue its collapse until a new source of pressure, neutron
degeneracy
pressure, is able to stop the collapse. This theoretical
prediction is
strongly supported by observations: no white dwarf has been found
with a
mass greater than 1.4 .
In fact, the vast majority of white dwarfs we see are nowhere close to the
Chandrasekhar limit. It seems that stars even with masses of 2-5
are able to lose enough mass in their planetary nebula stage to leave behind
white dwarfs of less than 1
. These white dwarfs are stable and just sit around for eternity, slowly cooling.
Given enough time, a large fraction of the mass in the universe will end up
in white dwarfs.
However, there are scenarios in which a white dwarf with a nearby companion can pull off material from that companion and slowly increase its mass. Such a companion may be a red giant or red supergiant which has a very weak gravitational grip on its outermost layers. If the white dwarf goes over the top, so to speak, the sudden collapse of the star can cause runaway fusion of its carbon and oxygen. The star becomes a huge nuclear bomb, a Type I supernova. It is thought that this kind of supernova was observed in 1572 by Tycho Brahe and in 1604 by Johannes Kepler. Unfortunately those are the last times that there have been supernovae visible in our Galaxy. (Note: The plural of supernova is supernovae).
Type I supernovae are distinguished by a lack of hydrogen lines in their spectra, and usually occur in old stellar populations (like elliptical galaxies). For this reason, the scenario I've described involving a white dwarf is thought to be appropriate, because WD's are not expected to have hydrogen. Later, we'll talk about Type II supernovae which do show hydrogen lines and occur in regions with lots of young stars (like spiral arms). These are thought to represent the explosive deaths of massive stars.
The nuclear reactions that occur during a Type I supernova can
produce
substantial quantities of heavier elements. Carbon ( 
C) and oxygen
( 
O) can
fuse to form silicon ( 
Si), and 2 silicon nuclei can
fuse to form nickel ( 
Ni). This is one of the ways that heavy elements
can by synthesized and ejected into the interstellar medium,
where
they can be used to form stars, planets, and life.
Start:Stars
